Wash lemon hot, grate dry, grate peel thinly, halve fruit and squeeze juice. Select blueberries, wash and drain well. Mix lemon zest, lemon juice, blueberries, 4 tablespoons sugar, 3 tablespoons flour and 1/2 teaspoon cinnamon.
For the crumbles, coarsely chop the almonds and dice the butter. Knead 150 g flour, 100 g sugar and butter. Add almonds and oat flakes and knead dough into crumbles. Put blueberries into a greased casserole dish (approx. 20 x 30 cm).
Spread crumbles on top and bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 40 minutes. Remove from the oven and dust with icing sugar if necessary.